The present invention relates to the field of temperature regulation devices and, more particularly, to a material containing gel particles that is useful for regulating fluid flow in response to changes in temperature such as the flow of water in a wet suit to control skin temperature of the wearer and to methods for regulating temperature using the same.
Exposure suits can be classified into two general categories, wet and dry. Traditional wet suits are usually manufactured of porous neoprene. They allow water to impregnate the suit material, trapping a thin layer of water between the fabric of the suit and the wearer""s skin, which is warmed by body heat. Essentially, wet suits rely on limiting the amount of water exchange between the inner surface of the suit and the environment to keep the wearer warm and, for this reason, suit fit is of critical importance. Generally, the degree of protection against the outside environment increases with fabric thickness. Although wet suits are efficient in protection against cold water, they are constricting because they have a tight fit and tend to inhibit free movement due to their thickness. Small punctures will not have a major impact on the functionality of the suit.
Dry suits, on the other hand, are manufactured of impermeable materials, normally have latex rubber seals at the extremities, and are filled with air. The trapped air provides an insulating effect in the suit, but thermal underwear must also be worn for protection against the cold. Although these suits allow ease of movement, they are not practical for swimming, because the buoyancy and displacement effects of the air hamper body control. If punctured, dry suits can take on significant amounts of water, rendering them useless in terms of insulation and dangerous in terms of flotation.
Survival and thermal exposure suits for ice water conditions tend to be of the dry suit type. These suits offer excellent protection by keeping the wearer dry and warm through a combination of the insulating properties of air and the use of underclothing. They are ideally suited to survival conditions because they provide a high level of buoyancy and the highest degree of thermal protection against cold conditions. They are, however, susceptible to leakage and punctures.
Once punctured and saturated with water, dry suits retain roughly 35% of their insulating capability. This asymptotic trend is attributed to the fact that once the suit is saturated, additional water simply fills the space between the suit and the body. At this point the only thermal protection is that offered by the waterlogged suit material. Based on the function of a wet suit, it can be argued that, should the dry suit not limit water exchange once punctured, the remaining insulation capability would drop further.
Tests have been conducted on individuals performing various activities in cold water while wearing wetsuits having different thicknesses. (See Wolff, A. H., Coleshaw, S. R. K., Newstead, C. G., Keatinge, W. R., 1985 xe2x80x9cHeat Exchanges in Wet Suits,xe2x80x9d Journal of Applied Physiology, vol. 58, no. 3, pp 770-777). It was reported that the flow of water between the suit and the skin was responsible for a significant portion of the heat loss in the suit. Typically, up to 30% of the heat loss at rest and up to 60% during exercise was reported to be caused by the forced convection resulting from water being pumped between the skin and the suit.
Consequently, it was reported that, for good insulation, fabric thickness is not as important as the limiting of water exchange under the suit. Wolff et al., reported that the difference in protection offered by a 4 mm and a 7 mm fabric is not radically different once the flow of water is restricted.
Even minor movement drastically increases the flow of water under the suit. The flow at the torso was reported to be roughly double that at the limbs, which can be attributed to the higher degree of suit fit accuracy on the limbs. In all cases, the test subjects were able easily to regulate and stabilize their body temperature for a large range of environmental conditions by controlling the flow through their movement. In cases where they overheated, simple movements would allow them to cool down by increasing water flow.
Various attempts at controlling the temperature of a diver have been made in the prior art. U.S. Pat. Nos. 3,367,319; 3,391,686; 3,402,708; 3,402,709; 3,450,127; 3,566,205; 3,572,314; 3,583,386; 3,884,216; 4,167,932 and 4,294,225 all disclose devices and methods for supplying heated fluid to a diver""s suit. Some of those methods rely on exothermic chemical reactions that result from the mixture of two substances. Others rely on the transfer of heat from combustion of fuel to water circulated within the diving suit. All of these devices require a power source, usually strapped to the back of the diver, which will function for a definite time before requiring replenishment. U.S. Pat. No. 3,430,688 discloses a garment that comprises a mesh of tubes in which a fluid is circulated to provide cooling to the wearer. U.S. Pat. No. 5,960,469 describes an undergarment to be used by divers with wet or dry suits. The undergarment has a number of bladders filled with an insulating fluid, which has very low thermal conductivity. Fluid from an external reservoir is pumped into the bladders to provide the desired level of insulation.
All of the aforementioned patents disclose attempts at regulating the temperature in garments for use in extreme environmental conditions, but none address the issue of how to control the temperature and flow of water due to pumping under the wet suit.
U.S. Pat. No. 5,722,482 to Buckley describes a fabric containing a material for use in the manufacture of wetsuits, where the material regulates skin temperature. This apparatus regulates heat transfer through the action of a phase change material, embedded in one or more layers of the fabric. The phase change material stores or provides latent heat energy through its transition between the solid and liquid states. The method seeks to control conductive heat transfer through the wetsuit fabric. Due to the nature of the phase change materials, the suit is effective only until the phase change of the available material is complete, after which the phase change material must be regenerated.
U.S. Pat. No. 5,277,915 to Provonchee et al. describes a gel-in-matrix composition containing a fractured hydrogel that was formed in a foam matrix. The gel is treated to form a network of fractured channels to create hydraulic permeability to allow aqueous media to flow freely through the gel-in-matrix for efficient, intimate contact of the gel with a liquid medium. There is no teaching or suggestion that liquid flow through the gel-in-matrix can be controlled by changes in temperature.
U.S. Pat. No. 5,447,689 discloses a method and apparatus for flow control that consists of sizing materials applied to a porous substrate, such as foam. These materials are able to hold a fluid for a discrete time and then release it, effectively delaying its flow, but they have no capability to regulate flow in response to prevailing environmental conditions such as, for example, a change in temperature.
The latent heat of phase change between liquid and gas has been widely used in the control of heat flux. That takes advantage of the substantially large amount of heat energy that can be stored as latent heat during phase changes. U.S. Pat. No. 5,955,188 to Pushaw discloses a foam substrate that is impregnated with a dispersion of microspheres having such a phase change material encapsulated therein and the method for producing it. The phase change disclosed in Pushaw is associated with latent heat energy for melting a solid material such as a paraffin and, thus, must be regenerated (solidified).
None of the prior art provides a method or a material for controlling fluid flow in a layer containing gel particles in response to changes in temperature.
The present invention provides a material useful for regulating the surface temperature of an object having its surface in contact with the material, wherein the material comprises a flow control layer containing gel particles and a protective layer. In a preferred embodiment, the flow control layer comprises layer comprises gel particles embedded in a foam substrate. The size of the particles is selected depending upon the response time desired for the gel to undergo a volume phase transition. Preferably, the gel particles have a particle size in the range of from about 1 xcexcm to about 1000 xcexcm. The material can be used to control the surface temperature of the object by the transfer of heat from the surface of the object to a fluid that can be absorbed by the gel particles. The gel forming the particles has a gel volume phase transition critical temperature that can be varied and that is determined by the application for which the material is used. Generally, the gel volume phase transition critical temperature (xe2x80x9cVPTCTxe2x80x9d) is approximately the same as the temperature that is desired for the surface of the object. The VPTCT is characterized as the temperature at which a material (the gel) begins to exhibit an accelerated change in volume of greater than approximately 1% per degree C. in response to a change in temperature of the material. For highly ionized gels, this acceleration can be represented as a discontinuity on the volume-phase transition curve for the gel.
Certain polymers known in the art are characterized by an inverse solubility behavior in aqueous solution. In the case of poly(N-isopropylacrylamide) (PNIPAM) that behavior is attributed to the delicate balance between the hydrophilic properties of the amide groups and the hydrophobic character of the isopropyl moieties. When heated above a lower critical solution temperature (the VPTCT), at which the hydrophilic PNIPAM chains collapse, the hydrophobic groups are accommodated within the separated polymer-rich phase. In the case of PNIPAM gels, such polymer chain collapse leads to a volume phase transition at temperatures slightly exceeding the VPTCT. On the volume phase transition curve, the VPTCT is characterized by the region of large rate of volume change versus small temperature change. This rate can be so large that the curve may exhibit a discontinuity in the region of the VPTCT.
The present invention also provides a wet suit article made from the above material having an outer water-permeable layer and an inner flow control layer, preferably made of gel particles embedded in an open cell foam matrix. The gel particles for this application have a gel phase transition critical temperature that is approximately the temperature at which the surface temperature of the wearer is desired to be controlled, e.g., in the range of about 18xc2x0 C. to about 23xc2x0 C.
The invention provides a method for controlling the flow of fluid in the material comprising a layer of gel particles embedded in a flow control layer, preferably an open cell foam matrix, wherein the gel expands when the temperature decreases to inhibit the flow of fluid in the material and contracts when the temperature increases to facilitate flow of fluid, thereby controlling the temperature. When the temperature of the fluid surrounding the gel particles has a temperature below the VPTCT, the gel particles absorb the fluid causing the gel to expand, reducing the permeability of the foam and reducing the flow the flow of water in the layer. In a wet suit, the expansion of the gel particles also provides a tighter fit to keep the wearer warm. When the temperature of the fluid surrounding the gel particles has a temperature above the VPTCT, the gel particles contract and expel fluid from the gel/foam matrix, thereby increasing permeability and permitting increased flow of fluid through the layer. This provides increased convective heat loss. In a wet suit, the expansion of the gel particles also provides a looser fit of the suit. In accord with the present invention, the gel undergoes a substantial volume change as a result of small changes in the temperature.
A wet suit material in accord with a preferred embodiment of the invention includes a composite laminate, which has at least one layer of an open cell foam in which is embedded approximately 5% to 80% mass fraction of hydrogel powder particles having a VPTCT that is approximately the temperature desired for the skin temperature of the wearer. The reversible property of the hydrogel particles regulates the skin temperature of the wearer, as described herein. The process is driven by the action of the hydrogel particles which react to the environment temperature in the presence of water by swelling and shrinking. The mechanism is driven by the pumping of water through the suit that naturally results from the diver""s movements and the skin temperature of the diver is controlled by the convective heat flux that results from flow of the water. Thus, the process is continuous, self-regulating and requires no external power source or recharging.
Preferably, a wet suit in accord with the present invention includes an outer protective layer, e.g., of neoprene, which covers the gel/foam layer. Such outer protective layer preferably also has insulating properties to inhibit heat conduction. The major factors influencing the thermal regulation ability of the suit are the cross-sectional area of the foam substrate, the permeability of the foam substrate, the VPTCT of the gel, and the response time of the gel (which is influenced by the particle size of the gel). The gel is tailored to undergo the volume phase transition at an optimal temperature for skin temperature regulation, shrinking above this temperature and swelling below it. This material provides a reactive, closed loop system that regulates the flow of water through the suit in submerged conditions by regulating the permeability of the foam substrate in response to environmental temperature through the volume change action of the gel that is due to the volume phase change in response to changes in temperature.
The operation of a wet suit made out of a material having the gel particle-containing layer in accord with the present invention is as follows. When a diver enters cold water, the cold water is absorbed into the suit and the flow control layer is flooded, cooling the gel. Consequently, the gel absorbs the water, thereby expanding to limit flow and tighten the fit of the suit. Because water is trapped in the gel, convective heat loss is effectively eliminated, minimizing the heat loss through pumping. Further, the neoprene also is a good insulator and, over time, in the absence of convection, the diver will begin to heat up. As the diver warms and the gel in the flow control layer is heated past its VPTCT, it will begin to shrink, thereby increasing permeability of the flow control layer. This results in increased fluid flow in the layer and resulting increased heat loss through convection, cooling the diver. Because the gel is formulated to have a VPTCT at a specific temperature, the process of heating and cooling will result in the average temperature of the flow control layer oscillating around the VPTCT, thereby maintaining the diver""s skin temperature at approximately the VPTCT, which is selected at a comfortable level.